Note that duvet is performed by the British group Bôa, not by the Korean artist BoA who performs in Japanese and English and is more easily stumbled on searching Japanese music with the letters 'b, o, a'. Both are worth checking out, just know who you're listening to. :)

Magia is the only END theme in my top-5, the rest are OPs.

I think my list is not particularly the best anime music around, but rather, it's the music that makes a lasting impression because it's such a good match to the anime that they are themes for. The themes continue to evoke the series long after it's been watched.

It's funny that the top two are from the same series, but both turned me into a fan of the respective artists.
As far as the songs from Angel Beats, I think it was a combination of well done music with absolute integration into the series itself that left such a huge impression on me.

Openings
Guilty Crown OP1 - My Dearest (sole reason I watched GC at first until I got into the story)
Code Geass OP5 - Worlds End (tough call between this and OP1)
Cowbpy Bebop OP - Tank (how can this not be on everyones list?)
Baccano! OP - Gun's & Roses
Spice & Wolf OP2 - Honey Coloured Dawn (won out over OP1 for the slow dancing in the intro)

Endings
Fairy Tail ED 13 - What You Gave Me (really mellow, love it in the episodes where it bled into the last few seconds)
Sword Art Online ED2 - Overfly (lovely solo piano opening on the full version, also how the ED makes sense when it falls into step with the series)
Hanasaku iroha ED1 - Hazy (starts mellow buts builds really well)
Spice & Wolf ED1 - The Wolf Whistling Song (so adorable, watched it so much I know all the lyrics)
Dont really watch the ED of series, really should as probably missed a lot of fantastic songs

So, do people think they would like the songs if they heard them on their own, or because of the visuals/emotions you recall from a series?